In virtually all systems that contain, pump, circulate, or process fluids (referred to herein generically as fluid processing systems) there exist regions that are expected to be full of either a liquid or a gas during at least some phase of normal operation. Often, these regions are subject to gas/liquid or liquid/gas transitions, either during normal operation or due to failure conditions. For example, there may be regions within the system that are expected to be filled with liquid during a first phase of operation, and devoid of liquid during a second phase of operation. Or there may be a region that is normally full of air and separated from a process liquid by a seal, but may become filled with process liquid if the seal fails.
Accordingly, it is sometimes desirable to include a sensor within a fluid processing system that can detect gas-to-liquid (GTL) and/or liquid-to gas (LTG) transitions within a certain region, either for control of certain processes or for sensing of unexpected conditions that might indicate a fault or failure within the system.
Apparatus for sensing the presence and absence of liquids in a monitored volume range from float-based physical sensors, radar-based sensors, ultrasonic sensors, vibrating sensors, and optical sensors. However, these approaches typically consume large amounts of physical space, require an external power supply that provides a high current or voltage, require calibration, and/or are limited as to the type of liquid that can be detected. They also tend to be prohibitively expensive for many applications.
Capacitance is sometimes used for measuring the level of a liquid that partially fills a monitored volume. However these devices tend to require long immersion depths, large connection sizes, and external power supplies. Also, they tend to be limited to sensing liquid levels only of a specified liquid or for a narrow range of liquid types. Accordingly, this approach is often unsuitable for applications that require small size, self-contained power, and adaptability to a wide range of liquids.
What is needed, therefore, is an apparatus and method for detecting LTG and GTL transitions in a monitored volume, where the apparatus is small in size, has minimal power requirements, and is modest in cost, and the method and apparatus are adaptable during implementation and use for accurate sensing of transitions involving liquids having a wide variety of dielectric constants, viscosities, temperatures, and conductivities.